Using External Monitor with Sony VAIO-VGNFZ290

I just installed OpenSuse11.0 (KDE 4.1) on a Sony VAIO Laptop ( VGN-FZ290). The graphics card is Nvidia GeForce 8400M GT.

The laptop screen resolution is 1280 X 800. When I hook up my external monitor to the laptop, I am unable to close the laptop without it going into sleep mode.

Is there a way to close the laptop or turn off the laptop display while the output is viewed on my external monitor?