I want to give Gnome-Shell the highest priority, but I couldn't change the priority on the System Monitor, an error messeage says: Can't change priority's process with pid 2841 to -5. Access denied

I tried the following commands:

gksu gnome-system-monitor

But I can't find Gnome-Shell on the processes list, so I tried "renice command", like this:

sudo renice -15 2841

And finally got this problem, the Gnome-Shell's PID always changes, it's not the same PID numbers, so I have to change the priority every time I start up my system.

Can anyone help me? Thanks.