I too use Dell XPS 15 and have suffered from over-heating in past when I used open source
nvidia-nouveau (installed from additional drivers).
Problem got solved when I started using
bumblebee where you can manually switch between intel graphics and nvidia graphics. Nvidia graphics gives much better performance but is also power expensive which results into overheating. Bumblebee gives you option to use only graphic heavy applications with nvidia graphics thus reducing overheating.
Guide to Install Bumblebee
After installation you can run any application with nvidia graphics by using
optirun <application-name> from terminal.
Also you can try
indicator-cpufreq which lets you put your profile to conservative, also reduce CPU frequency from applet-icon to prevent overheating when computational load is low.