Abstract: Due to growing concern on fossil fuel depletion and environmental pollution, integrated energy systems (IES) comprising electricity, natural gas and heat supporting multi-energy complementary gains increasing global research interest. Aiming at the precision of the model for IES energy flow calculation as well as the gas mixture problem brought by the increasing penetration of power -to-gas (P2G) in the future, this paper introduced the detailed mathematical models for P2G considering the effect of percentage of generated hydrogen and methane on the efficiency as well as electric heat pump (EHP) considering the effects of temperature conditions on its coefficient of performance, and investigates the impacts of mixtures of gas with alternative qualities on the natural gas system as well as the corresponding modeling methodology. Based on the traditional gas flow analysis, the method addressing the problem of gas mixtures is presented. Furthermore, a sequence iterative method based energy flow calculation of IES with mixture of gas with alternative qualities and detailed models of coupling components is proposed. Within the calculation, the load flow analysis of electricity system is conducted by Power System Analysis Software Package (PSASP). The effectiveness of proposed models and method is assessed by case studies.