A gas separation module includes hollow polymeric fibers held between a pair of tubesheets. The tubesheets are mounted to a core tube, and the distance between the tubesheets is maintained constant. The core tube is formed in telescoping sections, such that the fibers are attached to the tubesheets when the core tube is in its extended position, and the core tube is then collapsed, forming slack in the fibers. The core tube includes two distinct channels, connected to receive permeate and retentate gas streams, and to carry these streams to outlet ports while keeping the streams separate. Because the tubesheets are affixed to the core tube, the tubesheets do not move under the influence of gas pressure in the module. The slack in the fibers compensates for shrinkage of the fibers, prolonging the life of the module.