Summary form only given. This paper presents a new planning expansion model of an electricity and natural gas distribution system that has high penetration of distributed generation based on natural gas. The model entails lower investment costs as compared to traditional planning models that consider both systems (electricity and natural gas) as independent networks. This model is especially convenient for utilities that own both systems, as electricity and natural gas customers can derive benefits (via electricity or natural gas tariffs) if there are significant savings in investment costs. For fast-growing economies, the merging of electricity and natural gas distribution systems is seen as a promising business opportunity in which economies of scope can have lower investment costs, so there is a definite need for this type of approach.