This paper demonstrates, for the first time, a circuit-level approach to the analysis of pulse-ultrawideband (UWB) receiver front ends in the presence of interfering communication signals. The procedure is based on a model-order-reduction harmonic-balance technique based on Krylov subspaces. A sophisticated algorithm for performing matrix-vector multiplications has been particularly devised to handle signal spectra including very large numbers of arbitrarily spaced lines with very high numerical efficiency. The resulting simulation tool allows rigorous computation of interference effects on the nonlinear regime of UWB receivers and accurate circuit-level prediction of receiver sensitivity and channel capacitance. Simplifying assumptions typical of system-level approaches are overcome in this way, while keeping computational time at acceptable levels.