A process for polymerizing propylene is provided. The process comprises contacting propylene and optionally one or more monomers with a catalyst system comprising a bis-indenyl Group 4 metallocene compound supported on silica, the silica treated with one or more organoaluminum compounds and one or more heterocyclic compounds, under slurry conditions in the presence of hydrogen at a temperature of about 50.degree. C. to about 160.degree. C. and a pressure of from about 3 MPa to about 5 MPa to provide a catalyst activity of greater than 30,000 pounds of product per pound of catalyst; and then recovering isotactic polypropylene having a melt flow rate of 20 dg/min or less as measured according to ASTM D-1238 at 230.degree. C. and 2.16 kg.