Critical points of systems of ethylene with different polyethylene wax mixtures have been measured in the temperature range 413 to 513 K and at pressures up to 93 MPa. The results show a distinct influence of the polydispersity of the polyethylene wax on the critical point. A methodology for evaluating critical points of mixtures of ethylene and polydisperse polyethylene wax with a cubic equation of state and the Wong-Sandler mixing rule is proposed. We find that this method is able to predict critical points with good accuracy over a wide range of polydispersity. For industrial calculations it is advantageous to have a simple model with a limited number of parameters. This method applicates simple cubic equations of state to mixtures under high pressure.