The miscibility behaviour of polymer-nanoparticle / linear-polymer blends (all-polymer nanocomposites) has been investigated using an incompressible mean-field theoretical model that accounts for combinatorial, temperature-dependent exchange interaction energy and nanoparticle-driven effects. The theory is employed to predict the phase diagram of poly(styrene)-nanoparticle (PS-np) / linear-poly(vinyl methyl ether) (PVME) nanocomposites from room temperature to 675 K. Complete miscibility is predicted for PS-nanoparticles with radius < 6 nm blended with PVME (molecular weight 62 500 g/mol, nanoparticle volume fraction 20 %). The effect of PVME molecular weight and blend composition on the miscibility diagram is also addressed. When compared to the well-known experimental phase diagram of linear-PS / PVME blends displaying lower critical solution temperature (LCST) behaviour, the miscibility improving effect of sub-10 nm PS-nanoparticles is clearly highlighted. In terms of the model, this favourable nanoscale effect arises mainly from the reduced stretching induced by the sub-10 nm nanoparticles and the increased exothermic contacts when compared to nanoparticles with sizes > 10 nm.