ToME: the Tales of Maj'Eyal

I bought the game with a Steam key. Can I use that key on a non-Steam version of the game?

I'm not sure I understand what you want to do exactly.
If you have a steam key you must use this key in valve's steam client to get the game, just like any others.
Once you've done that if you want to play without installing steam you can, you just need to make an account on te4.org and bind your steam account with it.
Then jsut login with your login/pass from te4.org in the normal game client. Obviously this won't have steam integration

Thanks, that's exactly what I wished to do. Reason being I want to play the game without having the Steam client open because its blocked on this computer.