Although polymeric crystallization processes heterogeneously induced by foreign surfaces have been the subject of an overwhelming amount of experimental investigations over the last 30 years, this topic suffers from the absence of theoretical or predictive tools describing the polymer-substrate interactions at work in surface induced nucleation phenomena. In this work we consider the case when polymer chains are attracted to a flat surface (adsorption). Using a single-chain approach, we propose a model for deriving the thermodynamic potential barrier to nucleation within an adsorbed chain. It is shown that, because of the reduction of the configurational entropy of the entire chain prior to undercooling, forming a nucleus of a certain volume within the adsorbed chain costs less energy. In the case of weakly adsorbing surface, using the scaling treatment of de Gennes, we clearly relate the lowering of the thermodynamic potential for nucleation to the strength of the attractive interactions with the surface. This work points out adsorption as a possible mechanism for the nucleating activity of a surface.