If you don't want to be hot, just limit yourself to going on the gondola at night when there's no lightning. As long as the sun's gone, it won't be hot anymore, and they do have big windows. Or if you are going very short distances during the day, like from Pop to CBR or CBR to DHS. Those trips are only 3 minutes long, so as long as you don't get stuck, it won't hurt a thing. That's my plan.
I look at the big flap windows of the Skyliner today. They look promising, but not good enough fur a sunny day that's over 90 out. I imagine myself in there with my family and the possibility of it stopping in mid air...not gonna happen. If the buses go away, there's the Minnievan. However, I will take it to travel from Pop to CBR even on a sunny day around noon. It's only 3 minutes! Worth the risk!

And you can bet I will be timing it. I will also bring a thermometer with me.

In the winter, it should be much better, but I only go to WDW in the summer.
I am staying at Pop in September. I am imagining myself floating over to DHS each night to visit SWGE around 9:30 p.m and return around 10:15 (DHS will likely open till 10:30 p.m. or later). How neat is that?
Some people have told me that I should keep an open mind about the gondola. Trust me, if you have to use your mind to stay cool, you are in for it. The mind isn't that great at cooling! The Minnievan, on the other hand, has excellent air conditioning! Though there is a possibility that the gondola might shut down during the day, especially if it's over 90 out.
So here's my game plan:
During the day: use the Minnievan for DHS and Epcot, but do use the Skyliner for CBR.
During the night: use the gondola unless it is closed due to lightning. Then use taxi or Minnievan. Not gonna trust the replacement buses at that time, especially during park closings.