  In this paper, we examine the conditions under which the nonlinear transport
theory is inescapable, when a correlated quantum dot is symmetrically coupled
to two leads submitted to temperature and voltage biases. By detailed numerical
comparisons between nonlinear and linear currents, we show that the claimed
nonlinear behavior in a temperature gradient for the electric current is not so
genuine, and the linear theory made at the operating temperature $\bar{T}=
(T_H+T_C)/2$ is unexpectedly robust. This is demonstrated for the single
impurity Anderson model, in different regimes: resonant tunneling, Coulomb
blockade and Kondo regimes.
