You have a planet about the size of Earth. It needs to be continually dark, with just the slightest bit of light. Or no light at all. It makes little difference to the aliens that live there. But, this is mostly a swamp planet. The most important thing is that most of the planet doesn't dip below 32 degrees F. Or if it does, only for a short duration, and not too far. One definite possibility would be to have active volcanoes at the poles, but I'm worried that enough ash to block that much light would cause an ash induced winter, and I would prefer it if the planet didn't freeze.
The majority of stars are cooler and redder than Sol, and therefore emit a greater fraction of their light in the near infra-red, and less in the human visible band. Such a planet will appear dimly lit to humans. If it's close to the inner edge of the star's goldilocks zone and thermally stabilised by a high degree of cloud cover there may be places that experience continuous overcast or rain. That fits with the swamp scenario. Such a place will appear even dimmer to humans. Finally you might posit vegetation with black leaves because of a photosynthesis mechanism that soaks up far more human visible light than on earth. Because there is less usable light, foliage will be denser, targeting 100% coverage.
Caveats. Red stars are often flare stars and as such unlikely to harbour life. This one has to be otherwise. Also a planet around such a star is likely to become tidally locked to the star. You might get around this by making it a large moon of a gas giant planet instead. Finally native life is likely to evolve infra-red vision. It'll be dim to humans but not natives.
And I doubt there is any such thing as an all swamp planet. If it has weak or ceased tectonic activity there may well be no new high mountains, and large low lying swampy plains. But like that everywhere? No. At the very least there are oceans.
In the presence of thick enough atmosphere and some heat source (trapped radioactive heat or tidal heating), a planet can retain enough heat without any other external source. See the often cited article Possibility of Life-Sustaining Planets in Interstellar Space.
The conditions on the surface of such a planet will be rather un-Earth like, though.
short answer from me this time: place your planet in a brown dwarfs orbit. While it will heat up your planet up to the temperature you require if close enough, it wont emit.... wait, maybe it glows a bit... or... no, it won't.
Anyway, if you choose this one, you need a whole new evolution line, especially for plants, because there will be no visible light to do the stuff plants usually do (cracking CO2 into O2 and C (and adding some water and dead animals to make sugar out of the C)... but evolution is pretty... creative in its ways (if you can so so), so maybe you plant life using infra red light to do what these plants usually do (which is... another question).
if you add a thick atmosphere to keep the heat, you will get a pretty warm and maybe moisty planet, which may be counted as a swamp planet. But as nigel222 already noted, there is no "pure" swamp planet. Pure barren is okay, because there is no climate when you have no atmophere... but the have a swamp like place you'll need an atmosphere, which means you get a climate which means other regions on your planet may be less swampy.
For additional heat consider tidal forces. Friction can create much heat (just touch your cars breaks after stopping from 100mph to zero) and if you get a big moon it could add some surplus degree of warmth if your central brown dwarf isn't hot enough by friction heat your water bodys... well, that one degree... no, forget about this.
Another Idea, before I answer this, might be an enclosing nebula around your central star which does eat visible light but not infra red light. This... may need some more thinking I'm afraid.
EDIT: Just after clicking "answer" I started wondering about this. Solar winds would blow away such nebula when they are this close in the time it would take to spwan intelligent life at a planet orbiting this star. So do not consider this until you can explain why this nebula wont get blown away.
One to go: Your alien world is a secondary evolution. The first kind disappeared millenias ago; all they left is a dyson-sphere that eat all but the most red light to radio broadcast the first evolution guys favourite music into the space.
Between the high albedo of the higher clouds (reflecting 75% of all sunlight), and the cloud layer absorbing two thirds of the light that is not reflected, Venus is about as dim as an overcast winter day on earth.
And Venus is plenty hot. You can add lots of e.g. ash to the cloud layers without dropping below "swamp" temperatures.
So, all you need is
- dense clouds,
- of a composition that gives you lots of greenhouse effect,
- close enough to the main star.
Heat can be generated on a completely lightless world as "geo"-thermal heat caused by radioactive isotopes inside the planet and/or friction due to motion of a liquid interior caused by tidal forces.
If you moved Jupiter and all of its moons far enough from the sun and gave Io enough atmosphere to retain the heat generated in its interior, you would probably end up with something like what you are looking for.
From the above link:
This extreme geologic activity is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean satellites...
A lot of other answers have some good ideas, but there's one more that comes to mind:
Very large planet, therefore high gravity, high surface air pressure, therefore high temperature.
Pressure and temperature are directly related, but depending on what you're doing with the world, this pressure might be too high (E.G. humans would be very uncomfortable).
It could be a moon of a class L brown dwarf.
Some class L stars are large enough to support hydrogen fusion, and are red dwarfs. But there are many that are of substellar mass, and so are brown dwarfs.
A class L brown dwarf would be a deep red in color, and would shine very brightly in the infrared.
Any life that developed on the moon would make use of the IR extensively, and so it wouldn't necessarily be dim for them, but for any humans visiting it would be pretty dark.
The planet could be:
a) Far away from its sun. It could be heated by some nuclear reaction from the inside or from its moon. Maybe your aliens are non radiation-vulnerable? Maybe x-rays are what makes them see instead of normal light, that's why they don't mind the darkness.
b) Near the sun, but would be continually shaded by a second, larger planet on a closer orbit, with their orbits locked - the nearer planet would be heated to glowing, which could radiate enough heat and some light to your planet. The larger planet would protect your planet from instant scorching.
c) Orbiting a dying star