I have that problem on occasion, I know how frustrating it can be. One thing I`ve found that can exacerbate the problem, especially if its someone in the early stages of learning, is not tonguing properly (or at all). What happens is, the mouth becomes dry because of the action of blowing and the movement of the jaw which tricks the brain into thinking you`re eating, so on comes saliva, which builds up excessively - because youre not actually eating. If you focus on articulating more with the tip of the tongue, the rest of your mouth and your jaw will work less and your autonomous system won`t be triggered to produce too much saliva.
Another tip is to make sure you play with your chin up so the saliva drains backwards instead of into your mouthpiece. As for condensation, that should only happen when there is a big enough temperature difference between the metal and your breath. So.... keep a warm sax