Render certain passes only WITHOUT the combined I wish to de-noise my render using the compositor but I found half-way that I have no environment or emission layer selected. As rendering takes time, I wish to render only that two in order to be able to combine the passes again later on after I adjusted the noise of my indirect passes. 
Question is, how to render only some given passes without rendering the combined pass? 

EDIT

Or does it work if I just remove the tick from the combined option?
You should be able to get the result you want by unchecking "Combined" and just using Z and Emit. However! I strongly recommend that you first back up your existing render (and any other render layers) as 16-bit TIF images or EXRs incase of: Blender crashing during re-render, overwriting existing renders, etc.