The current design of a deep geological repository for high- and intermediate-level radioactive waste in France consists of a complex system of different underground structures (ANDRA, Dossier 2005 Argile, les recherches de l'Andra sur le stockage g,ologique des d,chets radioactifs A haute activit, et A vie longue, collection les Rapports. ChA cent tenay-Malabry, France, 2005). For a comprehensive understanding of the long-term hydraulic evolution of the entire repository, numerical non-isothermal two-phase flow and transport simulation, taking into consideration the generation, accumulation, and release of hydrogen gas and decay heat, are compulsory. However, a detailed numerical model of the entire repository system would require a tremendous computational effort and pose a laborious task with respect to the operation of the model. To handle these difficulties, we have developed an efficient method for the numerical modeling of a complete repository system and its geologic environment. The method consists of the following steps: (i) subdivision of the repository plane into a large number of "sectors" based on the position of hydraulic seals and on other geometrical considerations, (ii) exploitation of existing symmetries (inside or between sectors), (iii) adoption of the "multiplying concept", and (iv) connection of the individual sectors at the drift interfaces to form the entire repository model. Each sector is modeled as a three-dimensional (3D) block, and the entire model is computed with TOUGH2-MP. The method allows for a massive reduction in overall finite-volume elements and, at the same time, provides an adequate representation of the small-sized structures in the repository. The main characteristics of the method and its application to an entire deep geological repository system in a clay host rock are presented.