It works for me on version 9.0.1

Integrate[Exp[a*x]/(b - x), {x, 0, n}]

ConditionalExpression[

E^(a b) (-Gamma[0, a b] + Gamma[0, a (b - n)] + Log[b] - Log[a b] -

Log[b - n] + Log[a (b - n)]), Im[b/n] != 0 || Re[b/n] > 1 || Re[b/n] < 0]

Could you show what you actually typed? It is better to use lower case letters. Also "N" is reserved symbol in Mathematica.