  We report on the observation of magnon thermal conductivity $\kappa_m\sim$ 70
W/mK near 5 K in the helimagnetic insulator Cu$_2$OSeO$_3$, exceeding that
measured in any other ferromagnet by almost two orders of magnitude. Ballistic,
boundary-limited transport for both magnons and phonons is established below 1
K, and Poiseuille flow of magnons is proposed to explain a magnon mean-free
path substantially exceeding the specimen width for the least defective
specimens in the range 2 K $<T<$ 10 K. These observations establish
Cu$_2$OSeO$_3$ as a model system for studying long-wavelength magnon dynamics.
