Rather than using a heat gun, I have had good results using a domestic oven. Pre-heat the oven to 230C then place the motherboard in the oven component side up. I use a piece of cardboard to support the board in the oven. Heat for a couple of minutes only, you should be able to smell the flux at this time. Turn the oven off and leave for a couple of minutes, then gently crack the oven door open slightly and leave like this until cool. This sort of simulates the ramp up/hold/ramp down of the normal soldering process. I have reflowed a couple of Xbox 360 motherboards like this with good results.