  We carry out a systematic study of the thermal conductivity of four
single-layer transition metal dichalcogenides, MX$_2$ (M = Mo, W; X = S, Se)
from first-principles by solving the Boltzmann Transport Equation (BTE). We
compare three different theoretical frameworks to solve the BTE beyond the
Relaxation Time Approximation (RTA), using the same set of interatomic force
constants computed within density functional theory (DFT), finding that the RTA
severely underpredicts the thermal conductivity of MS$_2$ materials.
Calculations of the different phonon scattering relaxation times of the main
collision mechanisms and their corresponding mean free paths (MFP) allow
evaluating the expected hydrodynamic behaviour in the heat transport of such
monolayers. These calculations indicate that despite of their low thermal
conductivity, the present TMDs can exhibit large hydrodynamic effects, being
comparable to those of graphene, especially for WSe$_2$ at high temperatures.
