I will chime in here and add to what Mandi has said by saying that absolutely yes, heat intolerace is a classic symptom of MS.  When the heat increases, it slows down nerve transmission and increases the chance for having more MS symptoms. 

Some of the more common symptoms related to heat can include blurry vision, tremors, cognitive issues, and fatigue.  The symptoms are usually temporary and like Mandi has already said, heat does not cause a true exacerbation.  In other words, the heat will not cause you to get more lesions. 

Personally I have had most of my MS symptoms during the summer.  If I am out too long in the heat I begin to feel lightheaded, my gait changes, and a couple of times I had trouble with my speech.  I tend to listen to the weather report a lot to see what kind of a day I am in for and attempt to do anything outdoors in the early morning or in the evening when it might be a bit cooler.  Carrying around a water bottle filled with ice helps.  I don't go anywhere without my water. 

Thank you for your question.  Summer is almost over so hopefully we will all feel a little cooler and better.