An algorithm called POLYMIX is introduced for flash calculations of multicomponent systems containing polydisperse polymers. The polydispersity of polymers is taken into account through the concept of pseudocomponents. In industrial practice, a relatively high number of pseudocomponents are often required to represent multimodal polymer molecular-weight distributions. The algorithm formulation is independent of use of specific segment-based polymer equations of state for computing thermodynamic properties of polymer mixtures. The algorithm further offers the advantage that the computing time for polymer flash calculations is independent of the number of pseudocomponents used to represent the polydisperse polymer. Applications to polymer mixtures containing polyethylene and polypropylene are presented.