This paper presents a multiperiod generalised network flow model used to analyse the economic interdependencies of integrated energy systems comprising the electric network and the various fuel supply and delivery systems in a medium term operational time frame. By using a network flow programming model, one can take advantage of much faster solution procedures than standard linear programming techniques; an issue of importance considering the dimensionality of such integrated systems. The nodal prices that are obtained as a byproduct of the optimisation algorithm provide a way to analyse the economic interdependencies between the various fuel networks and the electric network. A numerical example is presented to highlight the benefits of the methodology and illustrate how nodal prices in the electric network are influenced by the dynamics of the various fuel networks.