We propose a methodology for systematic reactor design and optimization of operating conditions. The reactor can be designed systematically through the following steps: extraction of the chemistry from small-scale experiments, simulations for the virtual large-sized reactor configurations using the chemistry obtained from small-scale experiments, and final optimization by actual experiments on the real reactor manufactured according to the results of the simulation experiments. We call this technique ECONOMIX (Experiment, COmputer, kNOwledge MIXed CVD Process Design). The validity of this theoretical approach was confirmed for the WSix CVD process. The chemistry of the WSix CVD process was obtained from the tubular hot wall reactor. The rate constants of consecutive reaction were re-determined using two-dimensional simulation with consideration of temperature distribution. The growth rate and composition ratio were simulated for the cold wall reactor using reaction kinetics obtained by tubular hot wall reactor analysis. The predicted growth rates and composition ratios of Si/W showed good agreement with the experimental data. Therefore, the theoretical approach, ECONOMIX, is valid for the design of CVD reactor. We can design reactors and optimize operating conditions efficiently by this theoretical approach using computer simulation coupled with small-scale experiments.