A heat exchange apparatus of a high-load bottom-up-type condensing gas boiler is disclosed. In the gas boiler system having a ventilator attached to a lower part of a combustion chamber, the heat exchange apparatus structured in such a manner that a Venturi path for achieving a high-pressure and high-velocity flow of exhaust gas is mounted in an exhaust duct formed between a sensible heat exchanger in which first heat exchange is performed and a latent heat exchanger to which firstly heat-exchanged exhaust gas is flown, so that the sensible heat exchanger and the latent heat exchanger are perfectly independent from the exhaust gas after combustion. In addition, a curve-type exhaust guide and a V-guide are mounted to an upper part of the latent heat exchanger to guide a flow of the exhaust gas being firstly heat-exchanged and discharged from the sensible heat exchanger, such that condensed water generated during second heat exchange in the latent heat exchanger can be smoothly discharged and the thermal efficiency is enhanced.