I stopped inflicting Gnome on myself a long time ago, so I haven't tried it myself, but I did find this, whose relevant parts are:
- You need dconf-editor. Install it per your distribution's recommendations. I'm running Fedora 17 at the moment and this is the command line incantation I used: sudo yum install dconf-editor.
- Launch dconf-editor. On the left you will see a simple menu with five entries. Click on the plus sign to the left of org. New menu entires will drop down beneath it. Click on the plus sign to the left of gnome. Scroll down, then click on the plus sign to the left of mutter.
- Click on mutter next to the plus sign to highlight it. Several entries will appear in the large window to the right.
If the edge-tiling entry is checked, click it to uncheck it. If it is already unchecked, leave it unchecked.
- Scroll down the left panel and click on the plus sign to the left of shell.
- Go to the overrides entry below shell and click on it.
- Again, you are looking for the edge-tiling entry in the right window. Make sure it is not checked.
- Close dconf-editor.
Note: This turns off all edge-tiling. The Gnome Shell effect that snaps a window to the left or right side of the screen and opens it to one-half screenwidth will not work after disabling edge-tiling. If you decide to get it back, return to dconf-editor and re-enable it.