Is there a way to remove the dent/bend?
Since you didn't put a picture, I'm going to guess it is not significantly dented enough to worry about it (in which case, I'd just leave it alone since anything else will be uglier), but you should take it to a bike shop to get a professional opinion (so everything here is obviously all at you're own risk). 

One way is the ShelBroCo Dent Removal System (joke). A more serious overview from Sheldon Brown's site is here. 

For seat tubes and head tubes, you can try from the inside to press the dent out but I don't see you you could do it on the downtube. 

Frame blocks (made by companies like Bicycle Research, though you could make your own if you have access to a machine shop quite easily) are one solution. They're essentially 2 machined pieces of metal which you clamp over the tube and rotate the frame to try to push the tube back into shape. An example is here. Note that this will strip the paint off that area as well.

Bondo is another solution. 

The extreme solution is to replace the tube (or part of it).