Until now, I always wondered how to launch games with my dedicated nVidia graphics chip for Steam games on Linux.
Now I found a solution: Just put

primusrun %command%

into the extended start parameters for a game in your Steam library.

Primusrun has to be taken due to some 32-bitness of the respective games;
I furthermore tried running games via optirun which resulted in some weird and misleading error messages (e.g. that glGetError couldn’t be found which obviously can’t be true).