Marine seismic data acquisition with dual-sensor streamers (Geostreamer) enables separating the total wavefield in its up-going and down-going pressure and velocity components. The separation step is largely independent of the subsurface model and sea-surface shape. As a consequence realistic rough sea-surface shapes need to be included also in modelling studies and survey design. However, modelling of seismic data with rough sea-surface is not trivial and most of the standard modelling tools are still limited to flat or smoothly varying sea-surfaces. The scope of this thesis is to investigate including realistically rough sea-surfaces in PGS modelling software Nucleus+. From Rayleigh s reciprocity theorem in acoustic media, an integral expression for computing the down-going pressure wavefield from the up-going velocity wavefield and the sea-surface reflectivity is derived. The sea-surface reflectivity is computed based on Helmholtz-Kirchhoff integral using a free-surface boundary condition (Orji et al., 2011) and the up-going vertical particle velocity wavefield is computed as subsurface modelling from Nucleus+. The computation accuracy of the derived down-going pressure wavefield depends mainly on the numerical solution of the integrals and is proved by comparing with the total pressure field from Nucleus+ using smooth and flat sea-surfaces. Edge effects in the calculated down-going pressure wavefield are partly suppressed by using a cosine tapered window. Finally, the modelling is tested on a realistically rough sea-surface derived from a Pierson-Moskowitz (PM) spectrum. The total pressure field is computed by adding the computed down-going pressure wavefield including the effects of the realistic sea-surface to the modelled up-going pressure wavefield from Nucleus+.