If you actually burn wood/coal fires in that fireplace, I suggest that you don't do it. A plasma set generates quite a bit of heat itself and any extra heat in that environment isn't going to help the TV's long-term reliability. And if it's a solid fuel fireplace then you will occasionally get the puffs of smoke and ash that can find their way onto/into the TV, presenting another unfavorable factor.
If your fireplace is just for show or you have gas logs/gas stove vs. a "real" fire, then it might be a different story. A gas fire has neither the BTUs nor the residuals that you should be concerned about.
It is a gas fireplace.... the mantel is about 9" deep and the Plasma would sit about 3.5 to 4 feet above the fireplace with the mantel in between about 10" above the mantel. Is there a specific tolerance (temp wise) I should be looking for? Some people tell me to install a blower on the fireplace and then I would not need to worry.??