In this technical note, a model reduction procedure is presented for nonlinear systems that can be decomposed into a feedback interconnection of a linear and nonlinear subsystem. Conditions for stability of the reduced-order model and an error bound are given. Herein, the input-to-state convergence property is exploited, which proves to be useful in the definition and derivation of the error bound. The results are illustrated by application to a nonlinear mechanical system.