In my house I have a run of PEX tubing that runs right past, almost touching
in fact, the gas vent for the hot water heater. My concern is that the PEX
is in danger or melting or accelerated decay due to the heat. Is there some
way I can shield the PEX from the gas pipe, would there be a code or fire
reason why this particular section needs to be rerouted away from the
exhaust pipe?

Just to clarify the exhaust from the water heater would have traveled about
5 feet before reaching the point where it is near the PEX so I presume it
would have cooled down some.