An original methodology based on the use of experimental values of the conditional equilibrium constants in metallurgical calculations was employed to perform a theoretical modeling of the exchange interactions in melts formed during joint matte smelting of oxidized saprolite nickel ore and a product (cinder) of partial oxidizing roasting of pyrrhotite concentrate. It is shown that heating of the ore-cinder mixture (mass ratio ≈1.0) to 1,300°C will ensure the separation of the slag and matte phases with the optimal process parameters. The proposed smelting conditions were tested experimentally, the material composition of the products was studied by X-ray fluorescence and X-ray powder diffraction analyses, and the possibility of obtaining results close to those predicted theoretically was confirmed. By using a flux-free smelting (at 1,400°C) of the mixture, it became possible to convert at least 76.1% nickel, 79.9% cobalt, and 62.8% copper to matte, containing (wt.%): Ni — 8.4, Co — 0.39, Cu — 0.5, Fe — 56.8, S — 25.6, and O — 7.2. 75.3% of the matte phase consists of iron monosulfides, including nickeliferous pyrrhotites (Fe17.574Ni0.24S20 and Fe2.747Ni0.253S3). Cobalt is present as jaipurite (CoS), and copper is dissolved in pyrite (Cu0.25Fe0.75S2) and in metallic iron (Cu0.003Fe0.997). Low contents of non-ferrous metals in slag (0.24 wt.% Ni, 0.08 wt.% Co, and 0.04 wt.% Cu) are associated with the presence of significant amounts of metasilicates and free SiO2 in the slag composition. A sufficient matte separation factor (5.5 for nickel, 5.7 for cobalt, and 4.5 for copper) and reduced desulfurization (≈0%) confirm the selected optimal conditions of joint ore-cinder smelting. The applied methodology of theoretical modeling and the obtained results can be used in the development of technologies for processing mineral and technogenic raw materials with low copper and nickel contents.