Embodiments of the present application provide a method for manufacturing a polyolefin and a system for implementing the method. The method comprises combining a catalyst with a diluent mixture containing a diluent and an olefin monomer in a polymerization reactor. The diluent may comprise propane, butane, or isobutane, or a combination thereof. The polymerization reactor is operated at a pressure above a critical pressure of the diluent, but below the critical temperature of the diluent.