Is your Snow Leopard DVD a new DVD purchased from Apple?
Does the DVD surface appear clean, without scratches?
Have you tried simply blowing in the CD slot, with a can-o-air, etc?
You can also try a cleaning disk. (and you already tried that… )
But, the DVD drives in iMacs are not known to have live long, trouble-free lives. Try a couple of other DVD disks, just to see if that's a general problem with any DVD - or could be a bad installer DVD (it happens).
If other DVDs are also rejected, then you may need to replace that superdrive.
Much easier (and cheaper) is to find an external USB DVD drive, and use that instead of the internal.
Another option would be to burn a copy of your Snow Leopard DVD, on a USB flash drive. You would need to have access to another Mac, but you would end up with a more reliable, faster install from the flash drive, compared to the DVD.