The present disclosure provides oxidative coupling of methane (OCM) systems for small scale and world scale production of olefins. An OCM system may comprise an OCM subsystem that generates a product stream comprising C2+ compounds and non-C2+ impurities from methane and an oxidizing agent. At least one separations subsystem downstream of, and fluidically coupled to, the OCM subsystem can be used to separate the non-C2+ impurities from the C2+ compounds. A methanation subsystem downstream and fluidically coupled to the OCM subsystem can be used to react H2 with CO and/or CO2 in the non-C2+ impurities to generate methane, which can be recycled to the OCM subsystem. The OCM system can be integrated in a non-OCM system, such as a natural gas liquids system or an existing ethylene cracker.