Not practical. There are several problems.
First, what comes out of a rocket's rear end is mostly not even fuel.
Second, the big cloud of fuel you see upon launch is mostly water that prevents the rocket from tearing itself apart with its own shockwaves.
Last, but the most importantly, even if the exhaust was actually a new fuel that combusted with its parts at cold temperatures and there was much more of it, it would still not be practical. It all comes down to one question. Even if you catch fuel, how are you going to get that fuel, quickly moving in the opposite direction from which you are moving, all the way back, bring them into your fuel compartments, recondense them, and make sure there is no access stuff, before finally getting to reuse the fuel? It takes energy to bring the fuel back, it takes energy to pipe it back into the compartments, it especially takes energy to recondense your fuel back to liquid and make sure there is nothing that would completely block the fuel pipes in the engine.
All in all, it would take more energy than simply bringing more fuel, not to mention it would slow down your ship because it has to keep dragging around that heavy wire and fuel collector.