[en] This paper introduces the notion of discontinuity factors into the process of neutron energy condensation. The new condensation technique using few group diffusion theory exactly reproduces region-integrated reaction rates and leakage rates of a multigroup reference solution. The new technique is demonstrated by group condensation of a simple slab reactor from an eighteen group diffusion theory reference solution to a two group diffusion theory equivalent solution. To determine an exact slab geometry reference solution, development of a multigroup diffusion theory version of the analytical nodal method is first required. After this code development, the discontinuity factor method is verified to reproduce the region-integrated reaction rates and core eigenvalue of the reference slab reactor solution