The traditional way of determining the slip velocity of molten polymers is the classic Mooney technique which utilizes experimental data obtained from a capillary rheometer. However, measurements of the rheological properties of polymer melts in capillary flow at high shear rates are often complicated by viscous heating which is not taken into account by this mettled. A data analysis procedure based on a mathematical model for nonisothermal capillary flow of molten polymers is developed. Conduction, convection, and viscous heating are included together with the effect of wall slip. The technique provides detailed velocity and temperature fields in the die, and can be used to determine the slip velocity at high shear rates corrected for the effect of viscous heating. It is tested for the capillary flow of several polymers, including polystyrene, polypropylene, high-density, and linear low-density polyethylenes.