Numerical simulations of reactive flows are still a challenge when using full detailed kinetic mechanisms, which are becoming larger and larger to incorporate most of the combustion phenomena. One strategy to overcome this issue and simplify the treatment of the chemical kinetics is to use manifold-based methods, and one representative of this class is the Reaction–Diffusion Manifolds (REDIM). In the present work, the REDIM is applied for the first time to study soot formation in ethylene/air counterflow non-premixed flames. A simplified semi-empirical model for soot formation based on acetylene that describes nucleation, surface growth, and oxidation is used, and an uncoupled framework between gas phase and the formation of particulate matter is presented. Validation is carried out for laminar flames, and the results show that REDIM can predict with good accuracy the slow process of soot formation.