The human body has many different elements in its composition. Not all of those can become gaseous, or part of gas molecules, at room temperature. For example, the calcium in your bones, or the iron in your blood. We are constantly losing these through excretion and replacing them via food intake.
Some of the elements that do become gaseous at room temperatures are extremely poisonous and we would die if we excreted them this way, such as the chlorine which the stomach uses to make its acid. From the wiki:
Chlorine reacts with water in the mucosa of the lungs to form hydrochloric acid, destructive to living tissue and potentially lethal.
And could this water waste then be excreted through body heat?
In another question in this site, I did a calculation for how much energy a couple bacteria would release if their mass was completely turned into energy. The result is that it would release enough energy to increase the temperature of a human adult by around 1194 degrees Kelvin (approximately +2149F). A bacterium is considerably less massive than a droplet of water. I will leave the rest up to your imagination.