We generate a lot of heat, but so does the sun by shining on half of the earth's surface. That should not be a problem however, since it goes back into space in the form of infrared beams. The natural greenhouse effect obstructs just enough of those beams to keep our planet at a stable temperature that life has had plenty of time to adjust to. Now that the greenhouse effect has been artificially enhanced by adding lots of greenhouse gases to the atmosphere, more infrared beams (from the sun, but probably also some from human activity) are kept in the atmosphere and so it warms. So ultimately, the sun is just vastly more powerful than human activity, and any excess heat would be emitted into space if it weren't for the artificially raised greenhouse gas concentrations in the atmosphere.
