A tubesheet is formed at the end of a module containing polymeric material for gas separation, by immersing the end of the module in an epoxy material, or its equivalent. A vacuum, or partial vacuum, applied at or near the opposite end of the module, tends to draw the epoxy material towards the source of the vacuum, and results in a tubesheet having a desired thickness, even though the epoxy material may have relatively high viscosity. The method produces effective tubesheets, while minimizing the degree to which the tubesheet covers the otherwise useful surface area of the polymeric material. The method thus produces gas-separation modules having enhanced productivity.