An implicit method is presented to study the transient process of concentration distribution of multicomponent mixtures in separation cascades for gases. The problems that the first-order method (the so-called "transient approach") encounters have been overcome. The new method is second-order accurate in time and unconditionally stable. The nonlinear difference equations at each time step are solved by the q-iteration method. A simple cascade is studied as an example, and comparisons are made with the first-order "transient approach." The results show that the new method is advantageous over the "transient approach" in both accuracy and computational efficiency.