A method for the calculation of the γ-ray line-shapes observed in recoil distance Doppler-shift measurements of nuclear lifetimes is presented emphasising the case where a gate is set on a transition which feeds directly the level of interest. A description of such coincidence spectra is proposed for the first time. It is shown that a successful reproduction of the data requires to take into account the emission of γ-rays during the slowing-down of the ions in the stopper if the investigated lifetime is shorter than or comparable to the slowing-down time. The corresponding formalism for lifetime determination in the framework of the differential decay-curve method is developed. The new approach is illustrated by an application to experimental data obtained in the 110Pd( 28Si, 4 n) 134Nd reaction at a beam energy of 125 MeV.