The method used for theoretical investigation and optimization of multicomponent isotope separation by a gas centrifuge is described. The analysis of flow and diffusion for isotope mixtures is based on the numerical simulation of stationary axisymmetric equations for gas dynamics and convective diffusion under a centrifugal force. The possibilities of the technique developed are illustrated by the results of calculation the flow field and isotope mixture separation efficiency within the molecular mass range 40-240 of a process gas. The technique proposed can be useful in practice of stable isotope production for estimation the required number of separation stages in cascade and in creation of new types of centrifugal machines.