I also noted, in this necro-thread, that what SN3GUPPY said was-err-wrong.
Max ceiling of an aircraft can be EITHER minimum residual rate of climb (and what that value is can vary between customer and regulator) OR minimum buffet boundary.
We fly both GE and RR powered 777s. The RR versions have no buffet boundary problems right up to max altitude, as they run out of thrust before it becomes an issue.
The GE aircraft are exactly the opposite. They have no problem maintaining nearly 1000fpm up to there ceiling- which is defined by a minimum buffer between high and low speed buffet.