The development of accurate, flexible and robust dynamic reduced order models (ROMs) is an important step towards greater commercialization of entrained flow gasifiers (EFGs). This two-part series of papers describes the development of such a ROM. Part I, below, describes the model, while Part II presents its validation for four EFG designs and sensitivity analysis. The ROM employs a reactor network model (RNM) that approximates fluid mixing and recirculation using a series of idealized chemical reactors; well-stirred reactors and 1-D plug flow reactors. The RNM is constructed such that the ROM is capable of simulating the operation of a range of commercial or research gasifiers and syngas cooler configurations. The ROM incorporates submodels for multiple feedstocks, mixing and recirculation, particle properties, drying and devolatilization, chemical kinetics, fluid dynamics, heat transfer, pollutant formation, slag behavior and syngas cooling. It is the first experimentally-validated gasifier simulator to incorporate all of the above submodels as well as possess dynamic capability. Modifications to some of the submodels from their original form to improve ROM accuracy, flexibility and robustness are also discussed. The ROM performs the steady-state simulations presented in Part II of this series in 1-5 min on a desktop personal computer, compared to multiple days on multiple processors for more detailed CFD-based simulations. (C) 2011 Elsevier Ltd. All rights reserved.