The emergence of power-to-gas stations (P2GSes) has provided opportunities to make an efficient use of surplus power generated from intermittent renewable energy, thus linking the natural gas and electricity networks as an integrated energy system. To build new P2GSes, new electricity feeders and natural gas pipelines should also be planned coordinately to support the operation of P2GSes. This paper presents a game-theoretic planning model for the integrated energy system (IES) consisting of the natural gas system, electricity system, and P2GSes. P2GSes are assumed to be independent participants in the IES in a deregulated market environment. The Nash bargaining theory is employed to formulate this cooperative planning model for the first time. In addition, the profitability potential of P2GSes to provide emission reduction and secondary reserve services is innovatively and thoroughly evaluated in the proposed planning model. The proposed Nash bargaining planning model considering ancillary service is then applied to the IES comprising a coupled 24-bus electricity and 20-bus natural gas system. Case studies validate its effectiveness and we have compared the proposed model with two typical planning models including the traditional centralized planning and sequential planning models. The simulation results show that the proposed planning model can achieve the most fair and Pareto-efficient payoff allocation for the three independent participants and also achieve a good IES system performance. It can be concluded that our model can not only help to promote the popularization of P2G technology, but also enhance the cooperation among the P2GSes, the electricity system and the natural gas system. (C) 2019 Elsevier Ltd. All rights reserved.