The design and development of natural gas transmission pipeline networks are multidisciplinary problems that require various engineering knowledge. In this problem, the type, location, and installation schedule of major physical components of a network including pipelines and compressor stations are decided upon over a planning horizon with least cost goal and subject to network constraints. Practically, this problem has been viewed as a conceptual design case and not as an optimization problem that tries to select the best design option among a set of possible solutions. Consequently, conceptual design approaches are usually suboptimal and work only for short-run development planning. We propose an integrated nonlinear optimization model for this problem. This model provides the best development plans for an existing network over a long-run planning horizon with least discounted operating and capital costs. A heuristic random search optimization method is also developed to solve the model. We show the application of the model through a simple case study and discuss how non-economic objectives may also be incorporated into model. (C) 2007 Elsevier Ltd. All rights reserved.