The analysis of production systems in multi-reservoir gas fields is required to optimize production rates and enhance ultimate recovery. Conventional methods involve enormous computational efforts since production systems from reservoirs to a gathering station have been considered as one structure, and production rates are optimized step-by-step interms of objective functions. In this study, an new technique is developed to reduce the structural complexities of previous techniques and to consist of branch-systems including reservoirs and a surface branch-system without reservoirs. All branch-systems can be treated independently in this method. The decision variables including pressures and production rates in a branch-system, which are determined by the size of reservoir, the producing capacity of reservoir, and the initial pressure of reservoir, are updated simultaneously at every iteration step to accelerate its convergence. The method can analyze multi-reservoir gas fields with a fixed gross production rate at the gathering station efficiently. The results for two fields have shown that the method required less computational efforts and provided relatively accurate outputs for conventional techniques applied to two fields.