Which, it turns out, has very little to do with actual computers.
The intellectual skills involved in CS could, with not much difficulty, be turned to other kinds of problem solving such as operations research. Seriously, you're going to leave questions like how to most efficiently distribute scarce resources such as food to someone with a *business* degree? As a computer scientist, I'd create a model of the underlying problem, develop alternative algorithms, then show how those algorithms and model apply the real world problem. I use computer science every time I come home from grocery shopping. As I remove items from the bags I stage them by where they are eventually going to go. Why? Because efficient sorting algorithms eliminate lots of entropy early on. Consequently I only open my refrigerator *once*.
Computer science is essentially about figuring out the resources needed to accomplish things. If you want to figure out how much fodder it would take to move your draft animal powered army over a certain distance, you *could* consult a historian who specialized in the logistics of pre-mechanized warfare who'd tell you how Viscount Howe did it in the New Jersey Campaign of 1776-1777. Or you could find some CS graduate who pulled at least a "B" in algorithms to figure it out for you.
As for experts in gymel -- a technique for singing polyphony with one voice -- it's worth considering that the technique was developed in a period of human history that would be considered apocalyptically awful by modern standards. Even when times are violent, disordered, and desperately poor people still need art and music, and if we're stipulating that apocalyptic == "no computers", that means no iPods either. So it seems quite plausible to me that experts in gymel might find their services *more* in demand in a post-apocalyptic world.