The ambient temperature strongly influences the demand of heat and electricity in Finland. In order to study a potential expansion of the gas distribution network and the use of new gas sources, a mathematical model for the pipeline network was formulated. The model considers daily variations in the energy demand at the consumers, the gas flow rates and pressures in the pipes, as well as compressor duties. Using the model, the present natural gas system was simulated for a reference year. This gave a basic understanding about the gas flow rates and pressures in the pipes, as well as compressor duties for every day. The possibilities to extend the pipeline to other cities was also studied, and the implications for additional gas supply, using alternative sources, on the system was analysed, revealing an extra need of compression as well as information about the feasibility of the system at these new states. The pipeline network expansion problem was stated and solved as a nonlinear programming cost minimization problem, yielding information about an optimal operation of the network. Finally, an attempt was made to optimize the integration of a new LNG terminal from which regasified natural could be supplied to the pipeline.