Comment 4 for bug 1499929

I've just tested this with 15.04 with main + staging PPA. System does not suspend if "When plugged in" is set to "Nothing" with or without an external monitor connected (SVGA).

gnome-power-manager:
  Installed: (none)
  Candidate: 3.14.1-1ubuntu1
  Version table:
     3.14.1-1ubuntu1 0
        500 http://za.archive.ubuntu.com/ubuntu/ vivid/main amd64 Packages
     3.8.2-1ubuntu3~utopic1 0
        500 http://ppa.launchpad.net/gnome3-team/gnome3-staging/ubuntu/ vivid/main amd64 Packages

But then again, I don't have gnome-power-manager installed. Perhaps it's related to that package? Could you test without that package installed and let me know if it works?