Aqueous high molecular weight linear polyethylene nanocrystal/graphene composite dispersions were produced in situ by ethylene insertion polymerization in water or alternatively via a postpolymerization technique. Graphene was used without any prior compatibilizing surface modification. Remarkably, the Ni(II) salicylaldiminato catalyst employed is polymerization active in the presence of the functional groups of graphene in the absence of any scavengers or activating agents in the aqueous system. The intimate intermixing between the polyethylene nanocrystals and graphene in dispersion enables the formation of a uniformly dispersed graphene framework upon precipitation. Because of the very small size of the polyethylene nanocrystals, no segregation of the graphene on the particle surface is possible. The composites obtained feature a low electrical percolation threshold of the highly dispersed graphene. Commercially available graphene is compared with thermally reduced graphene oxide (TRGO) regarding its dispersibility in the polyethylene matrix in this novel water-based approach.