Decisions regarding natural gas production, processing, and transportation in interconnected networks depend on each other. Changes in the system affect both its capacity and flexibility. We discuss the main elements of the GassOpt model, an application developed to ensure optimal operation of this network by considering the complete system, its integration, and to provide valuable insights in the dependencies between the different system elements. The objective of GassOpt is to maximize the total flow of natural gas from the North Sea to the customers. The model determines the optimal routing, processing, and blending of natural gas ensuring that demand and quality constraints are satisfied. The application is used by natural gas producers and network operators to plan their activities, to evaluate the operational consequences of plans and investment decisions, and to maximize the value of their operations.