Using nonequilibrium Green functions, we calculate electronic and vibrational transport properties of disordered graphene nanoribbons. We show that edge disorder dramatically reduces phonon thermal transport while being only weakly detrimental to electronic conduction. The electronic and phononic elastic mean free paths points to the possibility of realizing an "phonon-glass electron-crystal". The calculated ZT values qualify zigzag graphene nanoribbons as a very promising material for thermoelectric applications.