Which are the multisensory brain areas? What would be an example of a multisensory brain area, where multiple senses (e.g. smell and taste) are combined to decode the corresponding stimuli from more than one sensory organ? 

I was thinking of association areas, but it doesn't really seem plausible that they would primarily be multisensory areas.
Association areas are exactly what you are looking for, actually.

For example, the ventral intraparietal (VIP) cortex, located in the inferior parietal lobe (along the right border of the yellow area in the image below), just on the border of the occipital lobe, integrates somatosensory (tactile) and visual information.



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For an extensive look at the single electrode studies which illustrated this convergence, see the following article:

Duhamel, J. R., Colby, C. L., &amp; Goldberg, M. E. (1998). Ventral intraparietal area of the macaque: congruent visual and somatic response properties. Journal of Neurophysiology, 79(1), 126-136. FREE PDF