1. In a digital processor, a method for simulating and optimizing a plant model, the plant model having a multiplicity of equipment models for a desired processing plant and a multiplicity of local property models for the material components within the plant, comprising the steps of: a) providing a set of initial values for each unknown variable in the plant model in a data storage area; b) determining by the digital processor a first set of coefficients in each local property model based on the set of initial values; c) storing the coefficients in the data storage area; d) executing simultaneously, in an outer level routine of a simulation routine, by the digital processor, a single, unchanging set of equations representing the equipment models and local property models, wherein said single set of equations is based upon a calculation of integration over time and includes a complementarity flash formulation, and wherein said single set of equations is simultaneously valid in all regions of a solution regardless of the state of the solution, by using the first set of coefficients to determine a second set of values for the plant model; e) storing the second set of values for the plant model in the data storage area; f) determining by the digital processor a second set of coefficients in each set of local property model; and g) storing the second set of coefficients in the data storage area.