NOVELTY - A system comprises: first heat exchanger adapted and configured to raise feed stream's temperature to temperature at or slightly above dewpoint of propylene; pump adapted and configured to pressurize feed stream to pressure of 400-500 psia; first membrane, where first membrane has feed port, residue port, and non-permeate port, and first membrane feed port is in fluid communication with pump; and second membrane, where second membrane has feed port, residue port and non-permeate port, and second membrane feed port is in fluid communication with first membrane non-permeate port.    USE - For membrane-based separation of propylene and propane feed stream (claimed).    ADVANTAGE - The current systems and methods allow membrane-based separation of propylene and propane, without a large change in temperature within or at a surface the membrane separator, thus allowing a permeate stream and residue stream to be withdrawn at or about the temperature of the feed stream. The current system is an improved membrane system for the separation of propylene and propane that uses fewer membrane modules and requires less capital cost. The higher pressure reduces the module count and the higher pressure-ratio (feed pressure to the membrane divided by permeate pressure from the membrane) improves separation performance. The evolution of heat by the condensation of the process feed fluid helps to keep the temperature constant and reduce capital cost and operational cost of the system. The current inventive process uses 1/3 more pump horsepower than the traditional gas separation approach.    DETAILED DESCRIPTION - A system for membrane-based separation of a propylene and propane feed stream comprises: a first heat exchanger adapted and configured to raise the feed stream's temperature to a temperature at or slightly above the dewpoint of propylene; a pump adapted and configured to pressurize the feed stream to a pressure of 400-500 psia; a first membrane, where the first membrane has a selectivity for propylene of at least 6.5, a feed port, a residue port, and a non-permeate port, and the first membrane feed port is in fluid communication with the pump; and a second membrane, where the second membrane has a selectivity for propylene of at least 2.5, a feed port, a residue port, and a non-permeate port, and the second membrane feed port is in fluid communication with the first membrane non-permeate port.