The curtailment of renewable energy would be reduced by converting it to hydrogen or methane using power to hydrogen (P2H) facilities or power to methane (P2M) facilities. Both hydrogen and methane can be injected into the existing natural gas system which has significant potential to unlock the inherent flexibility of integrated energy systems. The coordinated operation strategy of the hybrid power-natural gas energy systems considering P2H and P2M is proposed aiming to minimize the operational cost. In addition, a method to calculate the higher heating value of hydrogen-natural gas mixture is presented along with a strategy for handling the constraints of hydrogen mixture level limits. The simulation results of three case studies demonstrate the economic and environmental benefits of P2H/P2M in terms of reductions in cost, CO2 emissions and wind power curtailment. The differences in benefits between P2H and P2M have also been compared and analyzed. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.