Fuel cell power generation has evolved from the laboratory and aerospace applications, and moved onto practical applications of stationary power generation and automotive propulsion, driven by its high-energy efficiency and low emissions. The success of the fuel cell technology depends on its performance, cost, and reliability in commercial applications. Fuel Cell Energy Inc. (Danbury, CT) has been developing its direct fuel cell (DFC™) technology for power generation based on internal-reforming carbonate fuel cells. The DFC technology integrates the reforming reaction within the carbonate fuel cell stack. The integration of the reforming process inside the high temperature fuel cell stack simplifies the fuel cell power plant system and makes the fuel cell technology more accessible to the practical usage with low cost and high efficiency. The internal-reforming direct carbonate fuel cell technology has progressed steadily with improvement in performance and success in precommercialization applications. Modeling and simulation of the fuel cell performance played an important role in the fuel cell development. This paper will illustrate improved mathematical model for the direct carbonate fuel cell with the internal-reforming process and complete fuel cell physical and chemical descriptions for the simulation. The model has been validated with data from real-scale fuel cell stacks and applied to fuel cell stack design. More powerful and reliable DFC stack with improved performance has been developed with the assistance of this model. This paper will present progress in developing high performance stack designs aided by modeling efforts, its impact on power increase, and cost reduction in the DFC product.