Pumping Out Supergreenhouse Gases
Dichlorodifluoromethane, $CCl_2F_2$, or CFC-12, better known as Freon-12, alongside Sulfur hexafluoride $SF_6$ are two extremely potent greenhouse gases, thousands to tens of thousands of times more powerful, molecule for molecule, than CO2.
That means that you can get a similar effect with much smaller atmospheric concentrations. Instead of parts-per-million you can have parts per billion and get the same effect. On Earth, you have $2.996×10^{12}$ tons of CO2 in the air, so to get the same effect you'd need only a few hundred million tons of the super-greenhouse gas. Our civilization outputs 10,000 tons of $SF_6$ every year as a byproduct of industrial manufacturing. A determined campaign to create more could easily output truly industrial amounts into the planetary atmosphere in a few years.
Most of these gases have a lifetime in the atmosphere measured in the thousands of years, before they are disintegrated by reactions or by UV radiation, so unless you trigger a positive feedback, you'll have to have a generator forever outputting small amount of the gas to compensate for this loss-rate.
Ending the Ice-age
Since you specify that your planet starts off as a iceball wasteland with a narrow barely livable band around the equator, you need to do something about all that ice, since ice has a high albedo (reflects off heat) while ocean has a low albedo (dark blue absorbs better), so melting the ice would help push and keep the world outside of its ice-age. Additionally, being able to have more water vapor stay in the warmer atmosphere should contribute massively to the greenhouse effect (water vapor on Earth is reckoned to account for about 80-98% of the greenhouse effect, depending on whom you ask).
The cheapest way is to simply cover your glaciers with a dark layer at the top to remove the albedo issue. This way, you amplify the solar absorption many-fold. You might have to periodically redo this to compensate for runoff.
The next lowest level would involve building a vast number of fission reactors which are simply used to generate the largest amount of heat, with a pipe network gradually extended to melt the vast glaciers, in the same way some houses have electric bathroom tile-warming.
Finally, if you have basic space technology, you can deploy light-weight reflector mirrors, but they'd still be stymied by the ice albedo issue unless you already addressed that.
Dealing With the Mess
Your ocean levels will increase tremendously, your glaciers will destroy the landscape as they melt, and the permafrost ground will collapse as it melts and it's battered by the ocean and runoff. Even under optimistic scenarios, it'll be centuries before the earth settles into a new stable configuration.