How do I completely turn off screensaver and power-management?

For 11.10

Disable the GNOME Settings Deamon's power plugin from the command line:

gsettings set org.gnome.settings-daemon.plugins.power active false

Alternatively, using dconf-editor from the dconf-tools package:

For the screensaver:

gsettings set org.gnome.desktop.screensaver idle-activation-enabled false

Or again via dconf-editor:

You might find that the above solution is necessary, but not sufficient. The screen might still be blanking after 10 minutes. This is a default setting compiled into Xorg.

To disabled it for the current session, run:

xset s off

You can put this in your .xsession file to disable it upon every login.

You didn't specify if you want permanent solution or temporary solution. If you want to turn off screensaver and power management for e.g. 2 hours, you can simply inhibit those features.

In practice, you can simply run

systemd-inhibit sleep 2h

As an alternative, you may want to inhibit screensaver and power management until a given process has completed. For example,

systemd-inhibit wget "https://example.com/files/huge-download.dat"

would inhibit screensaver and power management until download has finished.

See man systemd-inhibit for more details. It allows you do disable only selected features and allows setting user visible string to explain why these features are inhibited (nice if your system has multiple users).