This paper aims to illustrate a new holistic modeling environment through a case study implementing a district heating system to supply four groups of users identified in the city of Tokyo. The holistic modeling environment, called DOME (Distributed Object-based Modeling Environment) is being applied by researchers in the Alliance for Global Sustainability (AGS) to develop a platform for assessing technologies and policies with the intent to reduce significantly CO2 emissions in large cities. DOME is used as an information based collaboration platform for supporting the decentralized integration of models built independently under various application tools and operating systems. It provides system engineers with the ability to access experts’ models over the Internet through user-friendly interfaces. The interfaces provide clearly identified input and output variables to support a facilitated integration. The DOME environment also provides tools for analyzing the resultant integrated systems. In this district heating simulation, a GIS (geographic information system) based land-use database manager, implemented as an object in DOME, is used to obtain the total ground area, the total building floor area, and the number and type of buildings in a specified region of Tokyo. Using the land use database manager, four groups of users are established. The heat demands of the users are derived using specific heat consumption data for the types of buildings considered. Users are either connected to the district heating system through direct heat exchangers on the supply or return line, or through local heat pumps which adapt the temperature to the needs locally, allowing a lower network temperature. Two different options are considered for the district heat supply: 1) a heat pump based superstructure comprising a gas turbine and a gas engine as cogeneration units, as well as an auxiliary boiler for peak load, and 2) a gas turbine / steam turbine combined cycle based superstructure with various option like steam injection and/or CO2 separation unit. Physical and costing models for the two alternative production unit superstructures and for the district heating system are connected with the GIS database using DOME. The environomic methodology, a simultaneous consideration of thermodynamic, economic and environmental criteria expressed as an aggregated cost, is used to assess the alternatives under different scenarios essentially characterized by the cost of natural gas, and the degree of polluion cost internalization.