A city's energy system processes, as well as the interactions of the energy system with other systems in a city are imperative in creating a comprehensive energy decision support system due to the interdependencies between critical segments of the system. City. Net is a sustainability-oriented decision support system that represents the energy, water, waste, transportation and building systems in a city while taking into consideration the integration and interdependencies that exist between these systems. This paper, which is focused on the integrated energy infrastructure system of a sustainable city, builds on the previous work which employs hierarchical decomposition and multi-domain formulation for the design of complex sustainable systems. The City. Net energy system encompasses the generation, transmission, distribution and consumption of energy in different forms, in several domains and at diverse scales in a city. Also, the interactions of the energy system with other aforementioned systems are incorporated in City. Net. The result is a scalable and flexible energy decision support system which can be simulated and used as a sustainability-analysis tool, encompassing environmental, social and economic sustainability.