Compared to the conventional thermal units and electrolytic devices, reversible fuel cells have very high efficiencies on both fuel cell mode of generating electricity and electrolysis mode of producing hydrogen or CHx. However, previous studies about fuel cells and its benefits of power to gas are not fully investigated in the electricity-gas energy system. Moreover, state-of-art studies indicate that hydrogen could be directly injected to the existing natural gas (NG) pipeline within an amount of 5%-20%, which are considered to make a slight influence on the natural gas technologies. This work proposes a novel electricity-hydrogen energy system based on reversible solid oxide cells (RSOCs) to demonstrate the future vision of multi-energy systems on integrating multiple energy carriers such as electricity, pure hydrogen, synthetic natural gas (SNG) and mixed gas of H-2-natural gas. The P2G processes of RSOC are sub-divided modelled by power to H-2 (P2H) and power to SNG (P2SNG). The co-electrolysis/generation processes and time-dependent start-up costs are considered within a unit commitment model of RSOC. The proposed electricity-hydrogen energy system optimization model is formulated as mixed-integer linear programming (MILP), where the H-2-blended mixed gas flow is linearized by an incremental linearize relaxation technic. The aim of the optimization is to reduce the energy cost and enhance the system's ability to integrate sufficient renewables through NG networks. Besides quantified the benefits of renewable level and H-2 injection limit on the P2G process, the numerical results show that RSOC combined with H-2/SNG injection results in productive economic and environmental benefits through the energy system. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.