Modular steel buildings offer rapid construction, quality control, and off-site fabrication, but the rapid loss of steel strength at elevated temperatures makes fire resistance critical. In modular systems, beams and columns are surrounded by layered fireproofing, insulation, cavities, and concrete slabs, so their temperature histories differ from individually tested members. Performance-based fire design should therefore be applied at the module level, rather than relying on member-level ratings developed for conventional steel frames. Current practice, however, often adopts such ratings unmodified, causing excessive protection thickness, reduced spatial efficiency, and limited planning flexibility. Previous work addressed this through a full-scale fire test on a representative modular unit per LPS 1501-1.1 and a coupled CFD–FEM model reproducing the observed behavior. While insightful, such analyses demand intensive time, cost, and expertise in fire dynamics and FEA, limiting their use in early design. Existing simplified formulas assume simple cross-sections with uniform protection and idealized heating, and cannot accurately predict steel temperatures in modular assemblies with composite slabs, discontinuous linings, and ventilated or closed cavities. To bridge this gap, the present study develops 2D heat transfer models for representative modular steel columns, ceiling beams, and floor beams, including steel, gypsum boards, insulation, and—for floor beams—concrete slabs, with temperature-dependent thermal properties and boundary conditions matched to gas temperatures from the full-scale test. A parametric study on steel thickness, fire protection thickness, cavity height, and slab thickness identifies the characteristic heat transfer mechanisms of each member type and quantifies the influence of governing parameters on maximum steel temperature and heating rate.