Combining cryogenic flash units with membrane units is attractive for gas separation processes such as effluent gas recovery and CO2 capture. However, systematic design methodology for such hybrid systems have not been reported yet. The hybrid system is a complicated system that not only includes separation units, but also includes pressure manipulation units and heat exchangers. A two-step decomposition strategy is proposed to deal with the design of such complicated system. The separation sequence is first determined with energy consumption consideration in the first step. Subsequently, the remaining energy recovery units, the heat exchanger network and turbines are simultaneously synthesized with the separation flowsheet optimization. When applying the proposed strategy to recover the effluent gas from polyolefin plant, we find a novel flowsheet that avoid intensive capital and operating cost by tightly integration of process units. (C) 2017 Elsevier Ltd. All rights reserved.